The present invention relates generally to an electric double layer capacitor, and more particularly to a high performance double layer capacitor made with low-resistance aluminum-impregnated carbon-cloth electrodes and a high performance electrolytic solution.
Double layer capacitors, also referred to as electrochemical capacitors, are energy storage devices that are able to store more energy per unit weight and unit volume than traditional capacitors. In addition, they can typically deliver the stored energy at a higher power rating than rechargeable batteries. Double layer capacitors consist of two porous electrodes that are isolated from electrical contact by a porous separator. Both the separator and the electrodes are impregnated with an electrolytic solution. This allows ionic current to flow between the electrodes through the separator at the same time that the separator prevents an electrical or electronic (as opposed to an ionic) current from shorting the cell. Coupled to the back of each of the active electrodes is a current collecting plate. One purpose of the current collecting plate is to reduce ohmic losses in the double layer capacitor. If these current collecting plates are non-porous, they can also be used as part of the capacitor seal.
Double layer capacitors store electrostatic energy in a polarized liquid layer which forms when a potential exists between two electrodes immersed in an electrolyte. When the potential is applied across the electrodes, a double layer of positive and negative charges is formed at the electrode-electrolyte interface (hence, the name xe2x80x9cdouble layerxe2x80x9d capacitor) by the polarization of the electrolyte ions due to charge separation under the applied electric field, and also due to the dipole orientation and alignment of electrolyte molecules over the entire surface of the electrodes.
The use of carbon electrodes in electrochemical capacitors with high power and energy density represents a significant advantage in this technology because carbon has a low density and carbon electrodes can be fabricated with very high surface areas. Fabrication of double layer capacitors with carbon electrodes has been known in the art for quite some time, as evidenced by U.S. Pat. No. 2,800,616 (Becker), and U.S. Pat. No. 3,648,126 (Boos et al.).
A major problem in many carbon electrode capacitors, including double layer capacitors, is that the performance of the capacitor is often limited because of the high internal resistance of the carbon electrodes. This high internal resistance may be due to several factors, including the high contact resistance of the internal carbon-carbon contacts, and the contact resistance of the electrodes with a current collector. This high resistance translates to large ohmic losses in the capacitor during the charging and discharge phases, which losses further adversely affect the characteristic RC (resistancexc3x97capacitance) time constant of the capacitor and interfere with its ability to be efficiently charged and/or discharged in a short period of time. There is thus a need in the art for lowering the internal resistance, and hence the time constant, of double layer capacitors.
Various electrode fabrication techniques have been disclosed over recent years. For example, the Yoshida et al. patent (U.S. Pat. No. 5,150,283) discloses a method of connecting a carbon electrode to a current collector by depositing carbon powder and other electrical conductivity-improving agents on an aluminum substrate.
Another related approach for reducing the internal resistance of carbon electrodes is disclosed in U.S. Pat. No. 4,597,028 (Yoshida et al.) which teaches that the incorporation of metals such as aluminum into carbon fiber electrodes can be accomplished through weaving metallic fibers into carbon fiber preforms.
Yet another approach for reducing the resistance of a carbon electrode is taught in U.S. Pat. No. 4,562,511 (Nishino et al.) wherein the carbon fiber is dipped into an aqueous solution to form a layer of a conductive metal oxide, and preferably a transition metal oxide, in the pores of the carbon fibers. Nishino et al. also discloses the formation of metal oxides, such as tin oxide or indium oxide by vapor deposition.
Still another related approach for achieving low resistance is disclosed in U.S. Pat. Nos. 5,102,745, 5,304,330, and 5,080,963 (Tatarchuk et al.). The Tatarchuk et al. patents demonstrate that metal fibers can be intermixed with a carbon preform and sintered to create a structurally stable conductive matrix which may be used as an electrode. The Tatarchuk et al. patents also teach a process that reduces the electrical resistance in the electrode by reducing the number of carbon-carbon contacts through which current must flow to reach the metal conductor. This approach works well if stainless steel or nickel fibers are used as the metal. However, applicants have learned that this approach has not been successful when aluminum fibers are used because of the formation of aluminum carbide during the sintering or heating of the electrode.
Another area of concern in the fabrication of double layer capacitors relates to the method of connecting the current collector plate to the electrode. This is important because the interface between the electrode and the current collector plate is another source of internal resistance of the double layer capacitor, and such internal resistance must be kept as low as possible.
U.S. Pat. No. 4,562,511 (Nishino et al.) suggests plasma spraying of molten metals such as aluminum onto one side of a polarizable electrode to form a current collector layer on the surface of the electrode. Alternative techniques for bonding and/or forming the current collector are also considered in the ""511 Nishino et al. patent, including arc-spraying, vacuum deposition, sputtering, non-electrolytic plating, and use of conductive paints.
The previously-cited Tatarchuk et al. patents (U.S. Pat. Nos. 5,102,745, 5,304,330, and 5,080,963) show the bonding of a metal foil current collector to the electrode by sinter bonding the metal foil to the electrode element.
U.S. Pat. No. 5,142,451 (Kurabayashi et al.) discloses a method of bonding the current collector to the surface of the electrode by a hot curing process which causes the material of the current collectors to enter the pores of the electrode elements.
Still other related art concerned with the method of fabricating and adhering current collector plates can be found in U.S. Pat. Nos. 5,065,286; 5,072,335; 5,072,336; 5,072,337; and 5,121,301 all issued to Kurabayashi et al.
It is thus apparent that there is a continuing need for improved double layer capacitors. Such improved double layer capacitors need to deliver large amounts of useful energy at a very high power output and energy density ratings within a relatively short period of time. Such improved double layer capacitors should also have a relatively low internal resistance and yet be capable of yielding a relatively high operating voltage.
Furthermore, it is also apparent that improvements are needed in the techniques and methods of fabricating double layer capacitor electrodes so as to lower the internal resistance of the double layer capacitor and maximize the operating voltage. Since capacitor energy density increases with the square of the operating voltage, higher operating voltages thus translate directly into significantly higher energy densities and, as a result, higher power output ratings. It is thus readily apparent that improved techniques and methods are needed to lower the internal resistance of the electrodes used within a double layer capacitor and increase the operating voltage.
The present invention addresses the above and other needs by providing a high performance double layer capacitor having multiple electrodes wherein the multiple electrodes are made from activated carbon that is volume impregnated with aluminum in order to significantly reduce the internal electrode resistance by decreasing the contact resistance between the activated carbon elements.
In one embodiment, the present invention can be characterized as a double layer capacitor, and method of making the same, comprising a capacitor case having a first part and a second part fastenable to each other to form a sealed capacitor case. The sealed capacitor case has a first capacitor terminal and a second capacitor terminal associated therewith. Also an electrode stack is contained within the sealed capacitor container. The electrode stack comprises a plurality of electrodes, each electrode includes a current collector foil and a carbon cloth impregnated with a specified metal in direct physical contact with the current collector foil. The current collector foils of alternating electrodes are coupled to the first capacitor terminal and the current collector foils of other alternating electrodes are coupled to the second capacitor terminal. A porous separator material is positioned between each electrode of the electrode stack. The porous separator material has pores therein through which ions may readily pass. The porous separator material prevents adjacent electrodes from electrically contacting each other. The electrode stack is maintained under a constant modest pressure within the sealed capacitor case. And a prescribed electrolytic solution is sealed within the sealed capacitor case, whereby the electrode stack is saturated and immersed within the electrolytic solution. In one embodiment, the porous separator material comprises a contiguous porous separator sheet that winds in between each electrode of the electrode stack in a serpentine manner.
In another embodiment, the present invention can be characterized as a wrapped electrode stack, and method of making the same comprising a plurality of impregnated carbon cloths, each having been impregnated with a specified metal; a plurality of current collector foils, each having a tab portion and a paddle portion; and a plurality of electrodes, each electrode comprising one of the plurality of current collector foils making direct contact with one of the plurality of impregnated carbon cloths. An electrode stack comprises the plurality of electrodes stacked such that alternating tab portions align with each other, forming a first set of aligned tab portions and a second set of aligned tab portions. And a contiguous porous separator sheet winds throughout the electrode stack in a serpentine manner between each of the plurality of electrodes and wrapped around the electrode stack, such that the contiguous porous separator sheet acts as an electrical insulator between adjacent electrodes of the electrode stack.
In a further embodiment, the present invention can be characterized as a double layer capacitor, and method of making the same. The double layer capacitor includes a capacitor case comprising a first part and a second part fastenable to each other to form a sealed capacitor case. The sealed capacitor case has a first capacitor terminal and a second capacitor terminal associated therewith. The double layer capacitor includes a first electrode comprising a first current collector foil and a first carbon cloth impregnated with a specified metal. The first current collector foil has a first tab portion and a first paddle portion. The first carbon cloth makes direct physical contact with the first paddle portion and the first tab portion is coupled to a first capacitor terminal. Also included is a second electrode comprising a second current collector foil and a second carbon cloth impregnated with the specified metal. The second current collector foil has a second tab portion and a second paddle portion. The second carbon cloth makes direct physical contact with the second paddle portion and the second tab portion is coupled to a second capacitor terminal. The first electrode and the second electrode are placed against each other, wherein a porous separator material separates the first electrode from the second electrode. The porous separator material wraps around the first electrode and the second electrode and acts as an electrical insulator between the first and second electrodes. The first electrode and the second electrode are compressed against each other with a modest constant pressure within the sealed capacitor case. And a prescribed electrolytic solution is sealed within the sealed capacitor case to saturate and immerse the first electrode, the second electrode, and the porous separator material with the prescribed electrolytic solution. Again, in one embodiment, the porous separator material may be a contiguous porous separator sheet that winds in between the first and second electrodes in a serpentine manner.
In yet another embodiment, the present invention can be characterized as a method of applying a modest constant pressure to an electrode stack comprising first providing an electrode stack. The electrode stack contains a plurality of electrodes, each electrode having a current collector foil and a metal impregnated carbon cloth placed thereagainst, and a contiguous porous separator sheet that winds throughout the electrode stack in a serpentine manner. A shim is placed against an exterior of the electrode stack, the shim having a specified thickness. And the electrode stack and the shim are inserted into a container, the container having an interior dimension less than the exterior dimension of the electrode stack having the shim placed thereagainst.
In yet another further embodiment, the present invention can be characterized as a method of ultrasonically bonding multiple foils together to form an electrical interconnection, and electrical interconnection formed, including the steps of: stacking a plurality of metal foils to be bonded together, each of the plurality of metal foils being coupled to an electrical device; positioning at least one dummy metal foil against the plurality of metal foils; and using a high frequency horn for ultrasonically bonding the plurality of metal foils and the at least one dummy metal foil together, the high frequency horn being directed at the least one dummy metal foil, wherein the plurality of metal foils remain intact and are bonded to each other and the at least one dummy metal foil.
In still another embodiment, the present invention can be characterized as a carbon cloth electrode for use in a capacitor comprising a carbon cloth comprising a plurality of twisted carbon fiber bundles that are woven together to form the carbon cloth. Each of the plurality of twisted carbon fiber bundles comprises a plurality of carbon fiber bundles which comprise a plurality of carbon fibers. Each of the plurality of carbon fiber bundles are twisted together such that an exterior of each of the plurality of carbon fiber bundles slightly frays to form the respective ones of the plurality of twisted carbon fiber bundles. The use of the plurality of twisted carbon fiber bundles reduces the transverse resistance of the carbon cloth electrode.